A nitride semiconductor device is used for high-power and high-frequency applications, due to its outstanding combination of fundamental physical properties, such as large band gaps, large breakdown fields, high electron mobilities, etc. In order to achieve the gain and efficiency requirements at K-band and above, the natural way is to achieve low gate capacitance by reducing gate length. However, GaN high electron mobility transistors (HEMTs) are vulnerable to short-channel effects like all field effect transistors (FETs) when gate length scales below 0.2 μm. The short-channel effects take the form of a drain dependent pinch-off voltage, the loss of saturation on output current including a linearity of a transconductance, a large off-state drain leakage current and low breakdown voltage.
Linearity of a transconductance is an important factor for RF power amplifiers due to the large dynamic range in the variable envelop of the modulation signals. An ideally linear FET device would possess constant drain transconductance over a wide range of input gate-source voltages. The problems in achieving flat transconductance are related to (1) small absolute distance between 2DEG and gate, (2) the increase of the access resistance at high drain current levels caused by quasi-saturation of the electron velocity, and (3) large transverse electric-field (E-field) perpendicular to the channel and the barrier/channel interface at the high current levels.
Thus, there is a need for a GaN material transistor structure that provides a high electron mobility channel having a linear transconductance with suppressed short channel effect for RF applications.